The present project originally was undertaken to test the hypothesis that the limbic system and striatal complex are sufficient, along with the neuraxis, to give expression to most forms of species-typical behavior. The striatal complex and the limbic system represent two evolutionary ancient systems of the brain, being derived, respectively, from reptiles and lower mammals. It was found that hamsters deprived of the neocortex from birth were capable of giving expression to the usual hamster-typical behavior. In the present experiments the operative procedure prevented the development of both the neocortex and the midline limbic cortex. Immature hamsters with these defects failed to show play-fighting, while adult females exhibited deficits in parental care.